This invention relates generally to the determination of physical parameters of piezoelectric resonators and, more particularly, to an apparatus and a method for determining the surface contour of substantially flat piezoelectric wafers.
Piezoelectric resonators are used for frequency control and selection in many applications, such as in monolithic filters for VHF radio apparatus. Filters of this type comprise a substantially flat piezoelectric wafer having an array of pairs of thin film electrodes deposited on the opposing faces of the wafer. The frequency response of the filters is principally dependent upon the geometry of the wafer, although it is also affected by the electrode shape, thickness, mass and alignment.
Currently, the manufacture of piezoelectric resonators typically includes a fine tuning step in which electrode mass is adjusted while resonator frequencies are monitored. Deviation in electrode dimensions, alignment of electrode pairs, electrode mass, and crystal angle as well as minor variations in wafer thickness may be compensated in this way, so long as the effects of variation in the wafer thickness are small compared to the effects of the variations in all of the other parameters.
However, it is becoming increasingly difficult to meet the very strict tolerances called for in many state of the art applications utilizing high frequency, high overtone resonators and multi-resonators. Also, with respect to the multi-resonator devices, in particular, there is a special need for a method for precisely determining the surface contour or thickness profile of the piezoelectric wafer in order to predict the relative frequencies of the resonators on the wafer.
Traditionally, piezoelectric wafer thicknesses have been determined by optical interference fringe techniques, which yield approximate thickness figures not useful for precise thickness determinations. Somewhat greater accuracy can be obtained using a more recent electrical technique in which the wafers are disposed between the electrodes of an air dielectric capacitor and the input impedance seen by this capacitor is measured. Since the resonant frequencies of the wafer vary inversely with the plate thickness, resonances observed at the capacitor terminals, using appropriate impedance matching circuits, are correlated to the plate thickness. However, an average, rather than a localized, thickness figure is obtained. Since the opposing faces of the wafer typically are not absolutely parallel but have a slight convexity, these thickness figures are related to the maximum thickness of the wafer, and are not helpful in predicting the localized thicknesses of the wafer, as required in high precision applications.